1. Technical Field
The present invention relates to a method for producing a polarizing element.
2. Related Art
As one type of polarizing element, a polarizing glass is known. A polarizing glass can be composed only of an inorganic substance, and therefore, as compared with a polarizing plate containing an organic substance, the deterioration thereof due to light is significantly less. Therefore, a polarizing glass has drawn attention as an effective optical device in a liquid crystal projector whose brightness has been enhanced recently.
As a general polarizing glass, those described in JP-A-56-169140 are known, and a method for producing such a polarizing glass is as follows.
(1) A glass product having a desired shape is produced from a composition containing silver and at least one halide selected from the group consisting of chlorides, bromides, and iodides.
(2) The produced glass product is heated to a temperature which is higher than the strain point but not higher than the softening point of the glass by about 50° C. for a period of time sufficient to produce crystals of AgCl, AgBr, or AgI in the glass product, whereby a crystal-containing product is produced.
(3) The resulting crystal-containing product is stretched under stress at a temperature which is higher than the annealing point but lower than a temperature at which the glass has a viscosity of about 108 poises so that the crystals are stretched to have an aspect ratio of at least 5:1.
(4) The stretched product is exposed to a reducing atmosphere at a temperature which is higher than about 250° C. but not higher than the annealing point of the glass by about 25° C. for a period of time sufficient to develop a chemically reduced surface layer on the product. By this process, at least a portion of the stretched silver halide particles are reduced to elemental silver.
According to the production method described in JP-A-56-169140, the halide deposits uniformly in the glass product, however, in the reducing step, only the halide in the surface layer of the glass product can be reduced, and therefore, the halide remains in a central portion in the thickness direction of the glass product. Due to this, the transmittance of the polarizing element is decreased.